The tractor vehicle and the trailer in the coupled-together condition form a load train or articulated train, wherein both the tractor vehicle and the trailer are configured with a boxlike, high-rising cargo space to maximize their transport volume. Especially when driving fast on a straight road, considerable vortices and turbulence occur around the gap between tractor vehicle and trailer, having a negative impact on the fuel consumption and the exhaust emissions. However, it is only possible to reduce the gap on a straight roadway, since on curved roadways there is a risk of the trailer colliding with the tractor vehicle. Accordingly, after reducing the gap during road travel it is absolutely necessary to restore a sufficiently large gap once again in event of a panic braking or an avoidance maneuver, while the term “automatic” means an adjustment of the gap internal to the system, that is, without human intervention.
In the past, efforts have already been made to reduce the gap during straight travel of the articulated train and to increase it when traveling on a curve in order to prevent a bumping of the opposite corners of the cargo space of the tractor vehicle and trailer. One generic prior art is formed by DE 33 33 231 A1 with a hitch that can be adjusted by means of a hydraulic slave cylinder, where the slave cylinder is fed from a master cylinder arranged underneath the rim bearing. As soon as the front axle of the trailer turns, the piston of the master cylinder is retracted or extended and it forces hydraulic fluid out from the master cylinder and into the slave cylinder, which in turn moves the tension lug in the axial direction of the hitch. The major drawback of this known hitch is that it only works with trailers having an articulated front axle and cannot be installed on trailers with rigid hitch. Another drawback is the exclusively angle-dependent control of the length-adjustable hitch, not taking into account the travel speed of the articulated train. Furthermore, this known type of constrained control generally works well as long as the geometrical relations of tractor vehicle and trailer do not change, that is, the trailer is always pulled by the same tractor vehicle. But if the trailer is pulled by different tractor vehicles with a chassis or cargo space contour differing from each other, there is a heightened risk of a collision of the trailer with the tractor vehicle upon sideways movement along a curve, due to too small a gap achieved between tractor vehicle and trailer.
A similar technical solution of a so-called short coupling system, which also enables the hauling of trailers with rigid hitch, is proposed by DE 41 36 334 C1, but here the slave cylinder located on the trailer is operated from a master cylinder arranged on the tractor vehicle, which in turn is activated by a mechanical gearing depending on the buckling angle between the tractor vehicle and the trailer.
It has become known from DE 195 07 034 A1 how to avoid damage to the forked draw-bar or mounted equipment of a tractor vehicle during backward movement of a truck with trailer. For this, a trailer coupling is provided with a coupling mouth, mounted so that it can swivel or be suspended on the cross beam of the chassis, and forced to swivel along with the forked draw-bar when the vehicle is steered accordingly. To detect the movement of the coupling mouth, sensors are arranged on either side of the opening of the coupling mouth. The sensors trigger an engine stop just before the forked draw-bar knocks against the chassis or other mounted equipment, so that the truck driver must first take other steps in order to operate his vehicle once again. The known truck draw-bar safety feature only works during slow maneuvering during backward movement and causes an engine shutdown when a given buckling between tractor vehicle and trailer is exceeded. This is a massive safety risk during fast highway driving and therefore it is not acceptable.